Particle contamination of surfaces is a concern in many areas of technology. Two areas where such contamination can be a very significant problem are optics, particularly those with critical optical surfaces, and electronic fabrication. The effect of contaminants on critical optical surfaces (coated or uncoated, dielectric or metal), for example in high power laser optics, can lead to increased optical absorption and a decreased laser damage threshold. Thus, as minute particles contaminate optical surfaces, they can serve as sinks for optical power incident on the optical surfaces and thus produce localized heating and possible damage. High power telescope mirrors, and space optics are other applications which require highly decontaminated critical optical surfaces.
In the electronics industry, particle contamination is an important factor in the manufacture of high density integrated circuits. Even in relatively conventional technology using micron or larger circuit patterns, submicron size particle contamination can be a problem. Today the technology is progressing into the submicron pattern size, and particle contamination is even more of a problem. Contaminant particles larger than roughly 10% of the pattern size can create damage such as pinholes which interfere with fabrication processes (such as etching, deposition and the like) and defects of that size are a sufficiently significant proportion of the overall pattern size to result in rejected devices and reduced yield. As an example, it has been found that the minimum particle size which must be removed in order to achieve adequate yield in one Megabit chip (which has a pattern size of one micron) is about 0.1 microns.
Filtration (of air and liquid), particle detection, and contaminant removal are techniques which are used in current contamination control technology in order to address the problems outlined above. For example, semiconductor fabrication is often conducted in clean rooms in which the air is highly filtered, the rooms are positively pressurized, and the personnel allowed into the room are decontaminated and specially garbed before entry is allowed. In spite of that, the manufactured devices can become contaminated, not only by contaminants carried in the air, but also by contaminants created by the processes used to fabricate the devices. Removal techniques for contaminants should provide sufficient driving force for removal but without destroying the substrate. Moreover, acceptable removal techniques should provide a minimum level of cleanliness in a reliable fashion. As the particle size decreases the particle weight becomes less significant as compared to other adhesive forces binding the particle to the surface which it contaminates. Removal of such small particles can potentially damage the substrate. In general, it has been found that submicron particles are the most difficult to remove. Many of the processes developed to clean integrated circuits, such as ultrasonic agitation, are not effective for micron and submicron particles and indeed, sometimes add contaminants to the substrate.